74 



much more than 100 feet from the surface. Wells of the Cali- 

 fornia type are frequently extended to depths of 1000, and 

 occasionally to depths of 2000, feet. Hydraulic rotary wells are 

 successful to depths of 1000 or 2000 feet in the proper rocks. 

 The percussion or churn drill may be used for all depths down 

 to 5000 feet or deeper if special outfits are provided. Diamond 

 drills have been successfully used to depths of 6000 feet. 



The Cost Factor. So many items, such as accessibility to 

 fuel, cost of labor, trade combinations, knowledge of water con- 

 ditions, relative availability of different drilling outfits and local 

 practice, enter into the cost of a well that only comparative 

 statements can be made. Instances are not uncommon where 

 wells of certain types have been put down for one-tenth the price 

 demanded for wells of the same type in other regions where con- 

 ditions are essentially similar. In general, however, if only 

 actual outlay of money is considered, the dug well is the cheapest, 

 for it may be constructed by the owner himself at times when he 

 has nothing else to do. Bored and driven wells do not require 

 expensive rigs and are often cheaper than dug wells when paid 

 labor is employed in their construction. The California type of 

 wells is moderately cheap in soft materials if the proper outfits 

 are available, but unfortunately their use is as yet confined 

 mainly to a single region. The jet process is adapted to the 

 sinking of a large number of adjacent wells in soft materials, 

 especially sand, and is occasionally successful for single wells, 

 although in most places driven wells can be put down at less cost. 

 The hydraulic and rotary processes may be cheaper than percus- 

 sion drilling where there are numerous alternations of hard and soft 

 material. Of the processes in use for drilling in rock the standard 

 rig (percussion drill) is the cheapest, the calyx and diamond sys- 

 tems being generally used only when cores of the rocks pene- 

 trated are desired. Further details of cost are given in Chapter 

 XVI. 



Comparative Safety of Types. - - The safety of a well depends 

 on the purity of the water at its source and on its protection 



